Hybrid vehicles have a brake system that is designed for recuperative braking. In recuperative braking, an electric motor of the hybrid vehicle, typically, the electric drive motor, is operated in generator mode. Following an intermediate storage, the electrical energy recovered during recuperative braking may be used for an acceleration of the vehicle. This reduces any power loss experienced by a conventional vehicle due to frequent braking during a drive. As a result, the energy consumption and the pollutant emissions of the hybrid vehicle are reduced in comparison to the conventional vehicle.
To ensure that the recuperative braking have as little effect as possible on the braking distance of the vehicle, it is necessary that the braking system of the vehicle be adapted to specific situations. For example, in the case of a full electrical energy-storage device, the braking system should generate the entire braking torque via the conventional brake, in particular, via at least one friction brake, since, in such a situation, the recuperative brake mostly does not exert any braking torque on the wheels.
Moreover, operating the electric motor in generator mode typically requires a certain minimum speed of the vehicle.
Therefore, a recuperative braking system is frequently not able to exert a braking torque on the wheels of a vehicle until the vehicle traveling ahead is at a standstill. Therefore, when a vehicle stops, the conventional brake system must compensate in the low speed range for the loss of braking action of the recuperative brake by generating a higher braking torque.
On the other hand, in many situations, it is desirable to exert a lowest possible hydraulic braking force on the wheels in order to achieve a high degree of recuperation. For example, following the gearshift processes, the decoupled generator often intervenes as a recuperative brake in order to shift the braking action again toward recuperative braking. If the total braking torque is to be kept constant in the process, then the conventional friction brake portion must be reduced correspondingly.
Processes where the braking torque of the conventional friction brake is adapted to the active braking torque of the recuperative brake in order to maintain a desired total braking torque, are often referred to as blending processes. In many vehicles having a recuperative brake, the blending is implemented by the driver actuating the brake input element. In the process, the driver assumes the task of deceleration controller. In the case of a discontinued or added recuperative braking torque, the driver uses the pedal to adapt the conventional braking torque in such a way that an overall deceleration desired by him/her is maintained. However, this process requires a greater expenditure of effort on the part of the driver.
Brake-by-wire brake systems, such as EHB electrohydraulic brake systems, for example, provide an option for controlling a total deceleration of a vehicle that is less work-intensive.
In the case of a brake-by-wire brake system, the braking torques can be blended by decoupling the pedal, without the driver having to additionally actuate the pedal or another brake input element. Therefore, in the case of a brake-by-wire brake system, the driver hardly notices the blending processes. However, conventional brake-by-wire brake systems are expensive due to the complex electronics and mechanics/hydraulics that they require.